Curable film-forming coating compositions have long been used in automotive and other industrial manufacturing fields. Curable compositions, that is, compositions that form hard surface protecting layers by chemical crosslinking offer better protection (as opposed to lacquers) to substrates because of their ability to resist damage by solvents, acids, and other chemicals, and effects of weathering. In order to cure by crosslinking, curable coating compositions contain chemical groups that are reactive under curing conditions to effect the required crosslinking.
In addition to the crosslinking functionality, curable crosslinking compositions contain compounds, usually polymeric, that include chemical groups selected for the properties they impart to the cured coating. Different types of polymers may be utilized in curable film-forming compositions, including polyesters, acrylics, polyurethanes, and polyepoxides. Each type of polymer contains specific chemical groups, providing unique properties to a cured composition. Urethanes are often desirable in coating compositions because they provide durability, hardness, and environmental etch resistance. A common way to provide urethane functionality in curable coating compositions is to use isocyanates as the crosslinking fumctionality, whereby the reaction products with polyols are urethanes. However, a disadvantage of this approach is that isocyanates are highly reactive and require careful handling and environmental control.